In the 1920s, the USPS placed giant concrete arrows every ten miles across the United States to guide pilots along the first transcontinental air mail routes. Hundreds still exist (though they've lost their bright yellow paint and night beacons). LOVE. And MUST FIND. Airmail Pilots, Giants Concrete, Transcontinental Air, Concrete Arrows, Airmail Route, Crosses Country Airmail, Coast To Coast Airmail, Yellow Arrows, Air Mail
This source is linked to another source. It has a bunch of great links for students to access. It gives them a look into different eras and how crazy/cool things were during that time. As a teacher, you could find a bunch of cool assignments or projects revolved around these resources.
Giant Concrete Arrows Across the American Landscape | Core77